A light-emitting element containing an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at a low voltage, is expected to be applied to a next-generation flat panel display or a next-generation lighting device. In particular, display devices having light-emitting elements arranged in matrix are considered to be superior to conventional liquid crystal display devices for their wide viewing angles and excellent visibility.
It is said that light-emitting elements have a mechanism of light emission as follows: by applying voltage between a pair of electrodes with an EL layer interposed therebetween, electrons injected from a cathode and holes injected from an anode recombine with each other in an emission center of the EL layer to form molecular excitons, and energy is released when the molecular excitons relax to the ground state; accordingly light is emitted. A singlet excitation state and a triplet excitation state are known as excited states, and it is thought that light emission can be achieved through either of the excited states.
An EL layer included in a light-emitting element includes at least a light-emitting layer. In addition, the EL layer can have a stacked-layer structure including a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, and/or the like, in addition to the light-emitting layer.
As a material having semiconductor characteristics, a metal oxide has attracted attention. Examples of such metal oxides having semiconductor characteristics are a tungsten oxide, a tin oxide, an indium oxide, a zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such a metal oxide having semiconductor characteristics is known (see Patent Documents 1 and 2).
A thin film transistor (hereinafter also referred to as a TFT) applying an oxide semiconductor has high field effect mobility. Therefore, a driver circuit of a light-emitting device or the like can be formed using the TFT.